Various abbreviations that appear in the specification and/or in the drawing figures are defined as follows:    AC access controller    AP access point    ARP address resolution protocol    CAPWAP control and provisioning of wireless access points    DHCP dynamic host configuration protocol    DNS domain name server    IP internet protocol    ISP internet service provider    MAC medium access control    STA station    vAC virtual access controller    WAN wide area network    WLAN wireless local area network
WLAN has evolved from merely providing local area coverage from a single access point to providing a coverage area that encompasses many access points. A WLAN may span over a large enterprise campus or possibly even an entire metropolitan area. In recent years architecture development has included a “thin” access point concept. The idea is to have one central controller that manages many access points. This facilitates management of the access point(s) and the overall WLAN feature set. The thin access point concept is implemented by splitting the termination of the IEEE 802.11 MAC such that part is terminated on the access point itself, and another part is terminated on the access controller.
As is shown in FIG. 1A, in the basic WLAN model a station (STA) 1 is associated with one access point (AP) 2. Each access point 2 can handle many stations 1. In this type of relationship the operation of handling many stations 1 involves accommodating STA-AP association, STA authorization, and STA-AP confidentiality. As can be appreciated, if the access point 2 is lost for any reason the service for the STA 1 is lost.
This conventional approach presents a number of challenges as the size of the WLAN is increased to include multiple access points 2. For example, forming large IP subnets results in the presence of a significant amount of broadcast traffic over the WLAN caused by ARP (ARP flooding). To solve a mobility challenge and the ARP flooding challenge one may use mobile IP and reduce the number of subnets. However, a disadvantage of this approach is that the STA 1 needs to have mobile IP implemented, and only IP-based communication is usable. Once the connection between the STA 1 and the AP 2 is lost it needs to be reestablished from scratch, resulting in long handover time.
As shown in FIG. 1B, in a conventional split MAC approach the station 1 has a relationship to the access point 2 and indirectly to an access controller 3. The state of association, authorization and confidentiality can be shared between the access controller 3 and the access point 2, or it may be solely owned and managed by either the access controller 3 or the access point 2. In an extreme case where the access point 2 owns all of these relationships the WLAN devolves to the conventional architecture of FIG. 1A. Some split MAC implementations propose to at least partially alleviate the ARP flooding problem through the use of spoofing, using the access controller 3 as the central point.
One challenge presented by this approach relates to scalability, as each access controller 3 can handle only some maximum number of access points 2. Beyond this limit other techniques such as mobile IP need to be used.
To summarize, the traditional split MAC concept relies on a 1:N relationship between the access controller 3 and the access point 2. However, the use of this approach has certain disadvantages. For example, as the size of the WLAN increases the processing power of the access controller 3 needs to also increase. Further, a loss of the access controller 3 results in a loss of the wireless service for all the access points 2, as the access controller represents a single point of failure. Another disadvantage is that this concept results in a bundling of the management plane and the user plane processing. Further, in some implementations this approach can transport user plane traffic directly to the Ethernet medium, which introduces broadcast flooding problems related to the creation of large subnets. And in practice most enterprise Ethernet switches are not capable of supporting more than 64K-128K MAC addresses, thus severely limiting the number of station 1 the WLAN network can support.
The IETF CAPWAP group is currently in the process of standardizing a split MAC transport protocol (see CAPWAP Protocol Specification, draft-ietf-capwap-protocol-specification-10, P. Calhoun et al. editors, Mar. 13, 2008). CAPWAP is focused on handling the access point 2 from a controller, and is related to the traditional split MAC approach discussed above.
There are different approaches to the traditional split MAC concept, e.g., where portions of the IEEE 802.11 MAC are terminated on the access controller 3 over an IP tunnel, and some over Ethernet.
When designing a large WLAN with, for example, 10,000+ stations 1 (end users) and 1,000 or more access points 2 it becomes difficult for these conventional approaches/proposals to scale up accordingly, both from a management plane and a user plane perspective.